Control of the Reversibility of Excited-State Intramolecular Proton

Article pubs.acs.org/cm

Control of the Reversibility of Excited-State Intramolecular Proton Transfer (ESIPT) Reaction: Host-Polarity Tuning White Organic Light Emitting Diode on a New Thiazolo[5,4‑d]thiazole ESIPT System Zhiyun Zhang,† Yi-An Chen,† Wen-Yi Hung,*,‡ Wei-Feng Tang,‡ Yen-Hao Hsu,† Chi-Lin Chen,† Fan-Yi Meng,† and Pi-Tai Chou*,† †

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan

‡

S Supporting Information *

ABSTRACT: By using the thiazolo[5,4-d]thiazole (TzTz) moiety as the core of a proton acceptor, compounds 2,2′-(thiazolo[5,4d]thiazole-2,5-diyl)bis(4-tert-butylphenol) (t-HTTH) and 4-tertbutyl-2-(5-(5-tert-butyl-2-methoxyphenyl)thiazolo[5,4-d]thiazol-2-yl)phenol (t-MTTH) have been strategically designed and synthesized. Upon photoexcitation, both t-HTTH and t-MTTH undergo a reversible type excited-state intramolecular proton transfer (ESIPT), the underlying mechanism of which has been verified by femtosecond early relaxation dynamics in various solvents. The pre-equilibrium in the excited state leads to both normal (∼440 nm) and proton-transfer tautomer (∼560 nm) emissions, for which the intensity ratio is dependent on both the molecular structure and the polarity of surrounding media. As a result, the emission can be widely tuned from blue to yellow via white-light luminescence. On the basis of t-MTTH, a white organic light emitting diode (WOLED) was successfully fabricated, which achieved external quantum efficiency (ηext) of 1.70% with Commission Internationale de L’Eclairage coordinates of (0.29, 0.33). More importantly, the electroluminescent spectra show superior color stability that is independent of luminance. The result demonstrates for the first time a credible WOLED based on a unimolecular ESIPT reaction, which may have far-reaching implications for practical application. relatively lower energy level,12 such that the emission ratio of blue (monomer) and yellow (excimer) in OLEDs is dependent on the driving voltage. Therefore, it remains important and challenging to search for white-light emission from a single emitting center, for which the electroluminescence spectral feature is independent of device parameters (e.g., driving voltage). On the basis of the above, an excited-state intramolecular proton transfer (ESIPT) system may provide a case in point.22−25 Most of the ESIPT reactions involve proton (or hydrogen atom) transfer from a preexisting hydrogen bond, which gives rise to a proton-transfer tautomer in the excited state. Theoretically, the reaction dynamics or thermodynamics of the unimolecular type ESIPT reaction can be harnessed via chemical modification or external stimulus (e.g., polarity), such that emission is from both reactant and product (protontransfer tautomer); hence, the overall luminescence properties may be fine-tuned to generate white light. Given that the criterion of these white light ESIPT systems lies in the existence

1. INTRODUCTION White-light emitting organic materials have drawn great attention recently owing to their potential applications in visual displays and lighting.1−4 White emission should ideally be composed of three (blue, green, and red) or two (blue and yellow/orange) primary colors to cover the entire visible spectrum (400−760 nm). To date, a large number of relevant materials have been developed via physically blended5−8 or chemically composited9−11 fluorophores with complementary emission colors. Recently, white-light emission from a single molecular entity has been proposed and developed due to several apparent advantages such as greater stability, better reproducibility, absence of phase separation, and simpler fabrication processes. Excimers,12−15 multinuclear complexes,16−19 or frustrated (partial) energy transfer systems10,20,21 have been designed for such a purpose. Despite being dubbed a single type of molecule, however, most of them still rely on the concentration effect or the combination of two or more emitting centers. Moreover, a common problem in, for example, excimer-based white organic light emitting diodes (WOLEDs) is the significant change of electroluminescence spectra with luminance, which is detrimental for practical application. The reason is that the excimer is usually in the © 2016 American Chemical Society

Received: November 3, 2016 Revised: November 13, 2016 Published: November 15, 2016 8815

DOI: 10.1021/acs.chemmater.6b04707 Chem. Mater. 2016, 28, 8815−8824

Article

Chemistry of Materials

the synthesis, the characterization, the underlying ESIPT mechanism probed by spectroscopy and femtosecond dynamics, the host-polarity fine-tuned ESIPT reversibility, and hence the WOLED properties are elaborated as follows.

of excited-state equilibrium, the energy difference between normal and proton-transfer tautomer states must be rather small and hence thermally accessible. In theory, this should suppress the driving voltage dependence and hence stabilize WOLEDs. Unfortunately, up to this stage, despite intensive research on the single ESIPT white light system,26−28 no meaningful WOLED has been attained, which hampers the next step toward practical applications. Recent fundamental advances on ESIPT molecules have revealed good empirical correlation among the acid−base property, hydrogen-bond (H-bond) strength, and thermodynamics properties of ESIPT.29 The results imply that lowering the basicity (acidity) of the proton acceptor (donor) leads to weakening of the H-bond and hence less thermally favorable ESIPT. Among the numerous ESIPT systems, the thiazole (Tz) compounds form an important class that has been intensively investigated.30−33 One prototype should be credited to 2-(2′hydroxyphenyl)benzothiazole (HBT, see Scheme 1), which is

2. RESULTS AND DISCUSSION 2.1. Synthesis and Crystal Structures. Compound tMTTM was obtained by reacting dithiooxamide and oanisaldehyde in dry dimethylformamide (DMF).43 As depicted in Scheme 1, t-MTTM was then subjected to demethylation by boron tribromide (BBr3) with appropriate molar equivalent (eq) to remove one or two methyl groups to give t-MTTH or t-HTTH, respectively. The full synthetic route is elaborated in the Supporting Information. All compounds were fully characterized with 1H NMR, 13C NMR, and HRMS. As for the 1H NMR study, the O−H proton peaks of t-HTTH and tMTTH were measured to be around 11.09 and 11.10 ppm in DMSO-d6 (see Supporting Information), respectively, which is significantly lower than that of HBT (δ ≈ 12.7 ppm) in DMSO-d6, supporting the lower basicity of TzTz than that of Tz and hence weaker intramolecular H-bond. The structures of t-HTTH, t-MTTH and t-MTTM were further confirmed by single crystal X-ray diffraction analyses. As shown in Figure 1 (t-HTTH and t-MTTH) and Figure S1 (t-

Scheme 1. Synthesis Routes of the Titled Compounds and Chemical Structures of HBT and TzTz

well-known to undergo ultrafast, highly exergonic ESIPT that results in solely a keto-tautomer fluorescence at ∼520 nm.33 Fundamentally, the analogue of the Tz moiety, namely thiazolo[5,4-d]thiazole (TzTz, see Scheme 1), has attracted our attention. TzTz is an important class of bicyclic aromatic molecule comprising two fused Tz rings that is widely used as the electron acceptor units for the donor−acceptor (D−A) πconjugated systems34−37 because of its high electron deficiency. The high electron deficiency of TzTz is expected to reduce the basicity of the nitrogen atom (cf. Tz). Accordingly, if Tz is replaced with TzTz as a proton acceptor, the associated ESIPT, if any, should be less exergonic than that of the Tz type such as HBT and may eventually reach equilibrium between reactant (normal species) and product (tautomer species) in the excited state. This, together with the good carrier mobility of TzTz,38−41 may lead to the fabrication of a practical WOLED. Standing on the above strategy, we report the design and synthesis of two phenol-substituted TzTz derivatives42 (see Scheme 1), namely 2,2′-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(4tert-butylphenol) (t-HTTH) and 4-tert-butyl-2-(5-(5-tert-butyl2-methoxyphenyl)thiazolo[5,4-d]thiazol-2-yl)phenol (tMTTH) to explore their ESIPT properties. For comparison, we also prepared 2,5-bis(5-(tert-butyl)-2-methoxyphenyl)thiazolo[5,4-d]thiazole (t-MTTM), in which both O−H protons have been replaced by the methyl group to prohibit ESIPT. Remarkably, highly efficient white light generation has been achieved from this novel TzTz class ESIPT system. As result, based on a single type of ESIPT molecule, WOLEDs have been successfully fabricated and achieved external quantum efficiency (ηext) of 1.70%, CIE coordinates of (0.29, 0.33), and superior color stability versus luminance. Details of

Figure 1. X-ray crystal structures of (a) t-HTTH and (b) t-MTTH with hydrogen bonding distances and twisted angles.

MTTM), the molecular structures of the three titled compounds are all nearly planar. Note that the planar structure of t-MTTM, in which no intramolecular H-bond is involved, manifests that elongation of π-conjugation plays a key role for the planarization. Careful examination of the crystal structures of t-HTTH and t-MTTH also indicates that there is no substantial π−π stacking due mainly to the addition of two bulkier tert-butyl groups (Figure S2). The introduction of the tert-butyl group is widely used in organic emissive materials and brings several advantages without significantly shifting the band gap. These include (i) increased solubility, which is a key factor for the later femtosecond fluorescence up-conversion study and device processing, and (ii) great reduction of the intermolecular contact. As a result, the emission can be free from the aggregation effect, which is particularly important because the ratiometric emissions of t-MTTH and t-HTTH are sensitive to the polarity of environments both in solution and in solid state. The negligible intermolecular contact allows fine-tuning of the emissions en route to white light by varying the host polarity (vide infra). 2.2. Photophysical Behavior. The steady-state optical properties of t-MTTM, t-HTTH, and t-MTTH in various solvents at room temperature are shown in Figure 2, and pertinent data are gathered in Table 1. All three studied 8816


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of t-MTTM, t-MTTH, and t-HTTH are located at 354/370/ 390, 364/380/401, and 369/390/411 nm, respectively. The trend of red-shifted absorption from none to one to two intramolecular H-bonds can be rationalized by the H-bond induced elongation of the π-conjugation44 and hence the lowering of the energy gap. As for the nonintramolecular Hbond and hence non-ESIPT model, t-MTTM exhibits a mirror imaged blue emission relative to the lowest lying absorption, with 0−0/1−0/2−0 vibronic transitions at 400/421/445 nm in cyclohexane (see Figure 2a). In stark contrast, for both tMTTH and t-HTTH, in addition to the mirror imaged normal emission band maximized at 440 nm, a large Stokes-shifted yellow emission emerges around 560 nm. The intensity ratio for the 440 nm versus 560 nm emission bands is independent of sample concentration. In addition, upon monitoring at 450 and 550 nm emission bands, as shown in Figure S3 of the Supporting Information, the same excitation spectra are obtained for both t-HTTH and t-MTTH, which are also identical to the absorption spectrum. These, together with the lack of a large Stokes shifted emission band in non-H bonded tMTTM, led us to unambiguously assign the 560 nm emission observed in t-HTTH and t-MTTH to the proton-transfer tautomer emission originating from ESIPT. As for t-HTTH, despite its possessing two −OH groups, only single proton transfer takes place. This is evidenced by the nearly identical tautomer emission in terms of spectral feature and peak wavelength. Moreover, according to the computational results, the tautomer corresponding to the second proton transfer cannot be located in the S1 state, which indicates that second

Figure 2. Steady-state absorption (in cyclohexane, dashed line) and photoluminescence (solid line) spectra of (a) t-MTTM, (b) t-HTTH, and (c) t-MTTH in various solvents at room temperature (CYH = cyclohexane, Tol = toluene, TCM = trichloromethane, DCM = dichloromethane) (λex = 380 nm). Note: the absorption and emission are normalized at peak wavelength of the normal species; the insets in panels b and c are photographs of t-HTTH and t-MTTH in solvents taken under UV irradiation at 365 nm, respectively.

compounds exhibit well-structured lowest lying absorption bands. In cyclohexane, the 0−0/0−1/0−2 vibronic transitions

Table 1. Photophysical Properties of t-HTTH and t-MTTH at Room Temperaturea steady-state measurement

t-HTTH

t-MTTH

solvents

λabs [nm] (ε/M−1cm−1)

λem [nm]

CYH

390 (92110)

Tol

time-resolved measurement Φf

τobs [ps] (pre-exp. factor)b

N: 446 T: 551

0.14

390 (88640)

N: 448 T: 557

0.12

TCM

390 (91367)

N: 447 T: 563

0.04

DCM

387 (95650)

N: 448 T: 564

0.04

CYH

380 (88430)

N: 433 T: 554

0.06

Tol

383 (89620)

N: 438 T: 558

0.07

TCM

380 (91042)

N: 438 T: 555

0.07

DCM

380 (91250)

N: 438 T: 560

0.03

450 nm: 1.7 (0.42), 268.0 (0.58)c 600 nm: 1.6 (−0.48), 268.0 (0.62)c 450 nm: 2.2 (0.69), 348.0 (0.31)c 600 nm: 2.1 (−0.43), 348.0 (0.57)c 450 nm: 1.6 (0.80), 232.0 (0.20)c 600 nm: 1.7 (−0.47), 232.0 (0.53)c 450 nm: 1.5 (0.89), 197.0 (0.11)c 600 nm: 1.3 (−0.53), 197.0 (0.47)c 450 nm: 1.6 (0.90), 417.0 (0.10)c 600 nm: 1.6 (−0.46), 417.0 (0.54)c 450 nm: 1.5 (0.92), 459.0 (0.08)c 600 nm: 1.4 (−0.45), 459.0 (0.55)c 450 nm: 1.3 (0.95), 392.0 (0.05)c 600 nm: 1.2 (−0.42), 392.0 (0.58)c 450 nm: 1.0 (0.97), 285.0 (0.03)c 600 nm: 0.9 (−0.58), 285.0 (0.42)c

τpt [ps] (kpt−1)

τ−pt [ps] (k−pt−1)

Keq

ΔE*d [kcal mol−1]

4.0

2.9

0.7

0.19

3.2

7.0

2.2

−0.47

2.0

8.0

4.0

−0.82

1.7

13.3

8.1

−1.24

1.8

16.0

9.0

−1.30

1.6

18.8

11.5

−1.45

1.4

26.0

19.0

−1.74

1.0

33.3

32.0

−2.06

a

CYH = cyclohexane; Tol = toluene; TCM = trichloromethane; DCM = dichloromethane; N = normal emission; T = tautomer (ESIPT) emission; kpt = proton transfer rate constant ; k−pt = reverse proton-transfer rate constant; Keq = kpt/k−pt, equilibrium constant. bObserved emission lifetimes (τobs) by femtosecond photoluminescence up-conversion. cPopulation decay time constant was applied for the fitting by subnanosecond timecorrelated single photon counting technique. dEnergy differences (ΔE* = E*tautomer − E*normal) between the normal form and tautomer form species in the lowest excited state (S1). 8817


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Chemistry of Materials ESIPT for t-HTTH is highly endergonic and thus thermally prohibited (vide infra). As shown in Figure 2 and Table 1, although the emission peak wavelengths (λem) of normal (N) and tautomer (T) luminescence bands of t-MTTH and t-HTTH are insensitive to the solvent polarity, the emission intensity ratio for normal versus tautomer emission is sensitive to both molecular structure and solvent polarity. In nonpolar solvent such as cyclohexene, t-HTTH, having a symmetrical structure, exhibits prominent normal emission (IN) and minor tautomer emission (IT) with IN/IT of ∼6.39, while t-MTTH, in which the TzTz derivative is slightly asymmetric, displays nearly equivalent normal and tautomer emission with IN/IT of ∼0.80. Except for dichloromethane, increasing the solvent polarity reduces the ratio of IN/IT substantially, with more tautomer emission for both t-HTTH and t-MTTH. A slightly high IN/IT ratio in dichloromethane (cf. trichloromethane) is rationalized by the shorter population decay time (see Table 1) of the tautomer emission and hence smaller quantum yield (vide infra). Clearly, these two systems exhibit ratiometric emission, in which changes of color from blue to yellow via white light as a function of solvent polarity can be detected with the naked eye as well as by the color hue index of the Commission Internationale de L’Eclairage (CIE) coordinates (see Figure S4 in Supporting Information). For instance, in dichloromethane, the CIE coordinates for t-HTTH were calculated to be (0.35, 0.36), which are close to the coordinates (0.33, 0.33) of standard white light illumination. Time-resolved measurements were carried out in various solvents to gain further insight into the excited-state dynamics of t-HTTH and t-MTTH. The results are shown in Figure 3

417.0 ps. On the other hand, the time-resolved tautomer emission monitored at 600 nm consisted of a 1.6 ± 0.1 ps risetime component and a decay time of 417.0 ps. The population decay time constants for both normal and tautomer emission bands were identical in t-HTTH and t-MTTH. Furthermore, within experiment error, the shorter decay component (1.7 ps for t-HTTH and 1.6 ps for t-MTTH) of the normal emission correlated well with the rise component (1.6 ps for t-HTTH and 1.6 ps for t-MTTH) of the tautomer emission. The results clearly conclude a precursor (normal form)−successor (tautomer) type of ESIPT in the early stage, followed by fast equilibrium between these two species prior to their respective emission (see Scheme 2). Indicated by the identical population Scheme 2. Reversible ESIPT Reaction Pattern for t-HTTH and t-MTTHa

a

[N*], concentration of normal species of excited state; [T*], concentration of tautomer species of excited state; kpt, proton transfer rate constant; k−pt, reverse proton-transfer rate constant; kN*, decay rate constant of N* for all decay channels except kpt; kT*, decay rate constant of T* for all decay channels except k−pt.

decay times for both normal and tautomer emissions (see Table 1 and Figure S5), similar pre-equilibrium is established for tHTTH and t-MTTH in toluene, trichloromethane, and dichloromethane. Under the existence of pre-equilibrium between the normal (N*) and tautomer (T*) species, that is, kpt and k−pt ≫ kN* and kT* (see Scheme 2), the time-resolved [N*]t and [T*]t can be expressed as shown (see eqs 1−10 in Supporting Information for details):28 ⎛ k −pt ⎞ k pt [N*]t = [N*]0 ⎜⎜ e−t / τ2⎟⎟ e−t / τ1 + k pt + k −pt ⎝ k pt + k −pt ⎠ [T*]t = Figure 3. Fluorescence up-conversion decay curves obtained for (a) tHTTH and (b) t-MTTH in cyclohexane. The data points (blue and red) shown are with the monitored emission wavelength as depicted. Solid lines depict the best biexponential fits. Inset: the enlargement of the early rise and decay curve up to 5 ps. The fitting parameters are summarized in Table 1. λex = 380 nm.

(τ1)−1 =

k pt[N*]0 (τ2)−1 − (τ1)−1

kN * + k T *Keq 1 + Keq

,

(e−t / τ1 − e−t / τ2)

(τ2)−1 = k pt + k −pt (1)

As a result, the equilibrium constant Keq = kpt/k−pt can be obtained by the ratio of the pre-exponential factor (at t = 0) in eq 1, corresponding to a free energy ΔE* from N* to T* (ΔE*= T*− N*). Because 1/τ2 is equivalent to kpt + k−pt, the forward and backward proton transfer rate constants can be further deduced and are summarized in Table 1. Several remarks can be pointed out according to these ESIPT dynamics (kpt, k−pt, and kpt + k−pt) and thermodynamics (Keq and ΔE*) data (Figure 4). The first point worth noting is the trend of the decrease in ΔE* in this series of ESIPT system when the solvent polarity is increased. For example, ΔE* of tHTTH is in the order of 0.19 kcal/mol (in cyclohexane) >

and Figure S5, and all pertinent data are listed in Table 1. For tHTTH in cyclohexane (see Figure 3a), the early time relaxation dynamics of normal emission (450 nm) exhibited a very fast decay component of 1.7 ± 0.1 ps and a long population decay component of 268.0 ps, while the tautomer emission monitored at 600 nm consisted of a 1.6 ± 0.1 ps rise time component and a decay time of 268.0 ps. As for t-MTTH in cyclohexane (see Figure 3b), the normal emission monitored at 450 nm also decayed biexponentially, which was fitted to be 1.6 ± 0.1 and 8818


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4b).22 Additional support of this viewpoint is given by the increase in the rate of ESIPT upon the increase in solvent polarity. For example, the forward proton-transfer rate (kpt = τpt−1, see Figure 4a and Table 1) increases in the order of (4.0 ps)−1 in cyclohexane < (3.2 ps)−1 in toluene < (2.0 ps)−1 in trichloromethane < (1.7 ps)−1 in dichloromethane for t-HTTH. A similar trend was also observed for t-MTTH. 2.3. Computational Approaches. Supplementary support of the above experimental results is provided by the computational approach using the TD-B3LYP/6-311+G(d,p) method, in which the polarizable continuum model (PCM) is incorporated for various solvents. Details of the calculation methodology are elaborated in the Supporting Information. Note that our aim with this computational approach is mainly to compare the relative thermodynamics between normal and tautomer excited states, and the calculation of reaction potential energy surface (PES), which may involve multidimensional reaction coordinates, is not the focus here. As a result, for both t-HTTH and t-MTTH, the enol-like normal form (see Scheme 2 was calculated to be the dominant ground-state species. Scheme 3 reveals that the electron density around the Scheme 3. Calculated Frontier Orbitals for t-HTTH and tMTTH of Normal form Involved in the First Singlet Excitation by TD-B3LYP/6-311+G(d,p)/PCM

Figure 4. (a) Plots of ΔE* (kcal/mol) and kpt × 1011(s)−1 for tHTTH and t-MTTH as a function of solvent polarity parameter Δf; Δf = (ε − 1)/(2ε + 1) − (n2 − 1)/(2n2 + 1), which are 0.00, 0.01, 0.15, and 0.22 for cyclohexane (CYH), toluene (Tol), trichloromethane (TCM), and dichloromethane (DCM), respectively. ε, dielectric constant; n, index of refraction averaged at 500 nm for the solvent. (b) A qualitative plot of free energy versus solvent polarization coordinate P based on the ESIPT coupled weak charge transfer reaction (see text). ΔE* is the energy difference between the N* and T* states. Note that the solvent induced potential energy curves are only qualitative.

−0.47 kcal/mol (in toluene) > −0.82 kcal/mol (in trichloromethane) > −1.24 kcal/mol (in dichloromethane). A similar trend for ΔE* is obtained for t-MTTH. In a qualitative manner, the plot of ΔE* as a function of the solvent polarity parameter Δf (see Figure 4 caption), shown in Figure 4, panel a, reveals a sufficiently straight line. The decrease of ΔE* as the solvent polarity increases indicates more stabilization of the T* species. In other words, the magnitude of the dipole moment of T* is expected to be larger than that of N* in their equilibrium polarization. Qualitatively, the above results can be rationalized by the canonical structure drawn for t-HTTH and t-MTTH, in which the symmetric dual moieties centered by the TzTz core mutually cancel out, in part, the dipole moment in the normal state. On the other hand, the occurrence of ESIPT leads to the enol−keto isomerization at one moiety, whereas the opposite site remains unchanged (see Scheme 2). The net result creates an uneven electron density distribution, that is, a consequence of charge-transfer, between two opposite moieties, which results in an increase of the dipole moment in the protontransfer tautomer (T*). Thus, T* is expected to be more stabilized than that of N* when the solvent polarity increases. In this case, amid ESIPT, we may essentially deal with the proton coupled charge transfer reaction. Assuming that the charge transfer coupling is weak, that is, in the nonadiabatic charge transfer region, the more stabilized energy on the T* (cf. N*) realizes the less solvent induced barrier (see Figure

intramolecular hydrogen binding site is mainly populated at hydroxyl oxygen and carbonyl oxygen at highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), respectively. The results clearly indicate that upon electronic excitation of t-HTTH and t-MTTH, the hydroxyl proton is expected to be more acidic, whereas the nitrogen atom is more basic with respect to their ground state, which drives the proton transfer reaction upon electronic excitation. Table 2 summarizes the computed emission wavelengths for normal and tautomer species, calculated by geometry optimization of the normal and tautomer S1 states, followed by executing the vertical S1 → S0 transition. Compared with the observed dual emission, the calculation slightly underestimated the emission gap for both normal and tautomer species. Nonetheless, the calculated energy separation between normal and tautomer emission is similar to that deduced from experimental data. Moreover, the trend of small changes for both emission peak wavelengths calculated in four solvents is consistent with the experimental results (cf. Figure 2 and Table 2). Table 2 also summarizes the computed energy differences between the normal and tautomer species in the lowest excited state for t-HTTH and t-MTTH. The differences in energy between tautomer and normal forms in the S1 state, ΔE*, were calculated to be −1.48, −2.13, −4.11, and −5.20 kcal/mol for t8819


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Chemistry of Materials Table 2. Calculated Optical Characteristics for t-HTTH and t-MTTH solvents t-HTTH

CYH Tol TCM DCM

t-MTTH

CYH Tol TCM DCM

computed λem [nm] b

N: 460 (446) T: 651 (551)b N: 461 (448)b T: 653 (557)b N: 463(447)b T: 659 (563)b N: 463 (448)b T: 664 (564)b N: 453 (433)b T: 607 (554)b N: 454 (438)b T: 607 (558)b N: 454 (438)b T: 608 (555)b N: 458 (438)b T: 612 (560)b

2.4. Luminescence in Solid Matrix. En route to the device application, we then investigated the optical properties of this new series of ESIPT system in solid matrix. Figure 5

ΔE* (kcal/mol)a −1.48 (0.19)c −2.13 (−0.47)c −4.11 (−0.82)c −5.20 (−1.24)c −1.32 (−1.30)c −1.67 (−1.45)c −2.84 (−1.74)c −3.58 (−2.06)c

a

Energy changes (ΔE*) calculated for the ESIPT reaction. bValues in parentheses are emission peak wavelengths. cValues in parentheses are calculated based on relaxation dynamics (see text).

HTTH and −1.32, −1.67, −2.84, and −3.58 kcal/mol for tMTTH in cyclohexane, toluene, trichloromethane, and dichloromethane, respectively. Though slightly different in terms of magnitude, qualitatively, the calculated trend of ΔE* (see Table 2 and Figure S6 of Supporting Information) is consistent with that of the experimental data (see Figure 4a for comparison), which decreases as the solvent polarity increases. As elaborated in the section on early relaxation dynamics, this result may be rationalized by the differences in dipole moment between normal and tautomer excited states. Using the same method (TD-B3LYP/6-311+G(d,p)/PCM), we then further calculated the dipole moments in the lowest singlet excited state, and the results are shown in Figure S7. The dipole moments of the N* species were calculated to be 0 D for t-HTTH, which was expected due to its symmetric dual chromophores, and 2.6 D for t-MTTH. In stark contrast, the dipole moments of T* species were calculated to be 24.9 and 19.3 D for t-HTTH and t-MTTH, respectively, for which the vector is pointed on the molecular plane (see Figure S7 in Supporting Information). However, it should be noted that the magnitudes of the dipole moments varied vastly with the different methods used for calculation. For example, with the TD-M062x/6-311+G(d,p)/PCM method, the dipole moments of T* species were reduced significantly to 8.4 and 2.8 D for tHTTH and t-MTTH, respectively. Nevertheless, despite the variation, the trend of a relatively large magnitude of dipole moment in the tautomer S1 state (T*, cf. N*) holds, which supports the conclusion that ΔE* increases its negative value upon increasing the solvent polarity. Finally, it should be worthy of note that t-HTTH provides two proton donating sites. Therefore, the possibility of excitedstate double proton transfer was investigated via computational approach. In this regard, we have made great efforts to locate the structure of any tautomer resulting from the double proton transfer in the excited state. Unfortunately, the corresponding geometry optimization could not be converged, which implies its much higher energy in nature. The results led us to conclude the occurrence of only single proton transfer in the lowest lying excited state for t-HTTH.

Figure 5. Emission spectra of spin-coated films containing 0.5 w/w% of (a) t-HTTH and (b) t-MTTH in PS (red), PC (blue), PMMA (cyan), and PAN (magenta) as well as their solid powder (black dash).

(dash line) displays the emission spectra of t-MTTH and tHTTH in pure solid state. For the solid t-MTTH, both normal (456 nm) and proton-transfer tautomer (572 nm) emission bands could be resolved, whereas solid t-HTTH exhibits dominant normal emission maximized at 460 nm. Given that ESIPT is slightly endergonic for t-HTTH in cyclohexane, the lack of tautomer emission may be rationalized by its even higher endergonic ESIPT due to the surrounding rigid, nonpolar media in solid. The fluorescence quantum yields (Φf) of t-HTTH and t-MTTH as pure solid were 26% and 20%, respectively, which are much higher than that in solution (cf. Table 1). This is mainly attributed to loose packing (see Figure S2 in Supporting Information) and the restriction of large-amplitude motions in rigid environments, which inhibit the rotation along the C1−C1′ axis (see Scheme 2). The polarity-sensitive effect in solutions for t-HTTH and tMTTH led us to further investigate their emission behaviors doped in solid polymers, that is, polystyrene (PS, Δf = 0.01), poly(bisphenol A carbonate) (PC, Δf = 0.05), poly(methyl methacrylate) (PMMA, Δf = 0.11), and polyacrylonitrile (PAN, Δf = 0.15). In this study, 0.5 w% t-HTTH and t-MTTH were used to keep their absorbance less than 0.5 at peak wavelength to avoid reabsorption. The results shown in Figure 5 indicate that the intensity ratio (IN/IT) for the normal versus tautomer emissions varied according to the polymer matrices, which resulted in changes of the luminescent color in the doped polymer films. This trend of IN/IT, decreasing as the polarity of the surrounding environment increases, was similar to that observed in the solutions. For t-HTTH in polymer matrices, the Φf values were 27% in PS, 24% in PC, 27% in PMMA, and 10% in PAN, and for t-MTTH, 34% (PS), 35% (PC), 35% (PMMA), and 20% (PAN). Upon being doped in the host materials, highly emissive dual emissions were resolved, and good performance WOLEDs were attained upon optimizing the host polarity, as elaborated in the following section. 8820


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Figure 6. (a) Device structure and materials used in this study; (b) current density−voltage−luminance (J−V−L) characteristics, (c) external quantum (ηext) and power efficiencies (ηP) as a function of brightness, and (d) normalized EL spectra of white devices using t-MTTH doped in different polarity hosts; (e) normalized EL spectra of t-MTTH based white devices by host TCB at various voltages.

Table 3. EL Performance of Devices

t-HTTH

t-MTTH

a

host

Vona [V]

Lmax [cd/m2]

DPEPO TCB mCP DPEPO TCB mCP

4.4 4.4 4.4 2.8 2.8 2.8

954 532 512 9310 9230 9160

(21.6 (21.2 (21.8 (13.6 (14.8 (14.4

V) V) V) V) V) V)

Imax [mA cm

−2

]

ηext max [%]

ηc max [cd A−1]

ηp max [lm W−1]

0.62 0.29 0.41 1.21 1.70 1.30

1.34 0.67 2.45 2.56 3.48 3.0

0.58 0.28 0.39 2.12 2.96 2.38

455 213 160 1300 1330 1340

at 100 cd cm−2b [%, V] 0.46, 0.05, 0.05, 0.77, 0.76, 0.61,

14 16 18.4 7.2 8 7.8

CIE [x,y] 0.26, 0.25, 0.24, 0.29, 0.29, 0.29,

0.36 0.41 0.38 0.31 0.33 0.35

Turn-on voltage at which emission became detectable. bValues of ηext and driving voltage of device at 100 cd cm−2 are depicted in parentheses.

2.5. OLED Property. t-HTTH and t-MTTH were applied as the emitters to examine the electroluminescent (EL) properties. In this approach, we used three different polarity materials, namely, bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO),45 1,3,5-tri(N-carbazolyl)-benzene (TCB),46 and m-bis(N-carbazolyl)benzene (mCP),47 as hosts to tune the intensity ratios for the normal versus tautomer emissions. Figures S8 and S9 show the Φf of t-HTTH and t-MTTH doped in different hosts. For the t-MTTH in host matrices, both normal and proton-transfer tautomer emission bands could be well resolved, whereas only normal emission (weak tautomer emission) was observed for t-HTTH in host matrices. The Φf was 13% (DPEPO), 15% (TCB), and 20% (mCP) for t-HTTH, and 19% (DPEPO), 23% (TCB), and 26% (mCP) for t-MTTH. To obtain the energy levels more closely related to the values in the solid-state devices, we estimated the energy levels of the HOMO levels by using photoelectron yield spectroscopy (Riken AC-2). On the other hand, the LUMO levels can be estimated by subtracting the optical energy gap from the measured HOMO. The corresponding HOMO/ LUMO energy levels of t-HTTH and t-MTTH were then calculated to be −5.16/−2.35 eV and −5.68/−2.79 eV, respectively (see Figure S10 and S11 in Supporting Information). We then fabricated WOLEDs in the structure of ITO/CzSi: 4 wt % ReO3 (60 nm)/CzSi (15 nm)/host: 30 wt % dopant (20 nm)/Bphen (50 nm)/LiF/Al. Figure 6, panel a shows the

molecular structures and energy level of the host used in the device. 9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi),48 having a suitable energy level (HOMO/LUMO = −6.0/−2.5 eV), can efficiently confine excitons and was selected as the hole-transporting layer (HTL). To lower the hole injection barrier from ITO to CzSi, we used rhenium oxides (ReO3)49 as a dopant material in CzSi to improve the hole injection efficiency. The emitting layer (EML, 20 nm), which consists of 30 wt % of the ESIPT dopants dispersed in the host matrix (DPEPO, TCB, and mCP), was subsequently implemented through vacuum deposition. To further confine the holes or the excitons generated within the emissive region, 4,7-diphenyl-1,10-phenanthroline (BPhen), which has a highenergy gap (HOMO/LUMO = −6.4/−3.0 eV), was selected as the electron-transporting layer (ETL). Lithium fluoride (LiF) was used as the electron-injection layer, and aluminum (Al) was the cathode. Figure 6, panels b−d depict the current density−voltage− luminance (J−V−L) characteristics, device efficiencies, and EL spectra of the device in which t-MTTH was used as the dopant. The EL characteristics with t-HTTH employed are shown in Figure S12 in the Supporting Information. The pertinent data are summarized in Table 3. Taking into account the difference in the J−V performance caused by using different dopants, tMTTH devices exhibited a clearly lower turn-on voltage (2.8 V) and higher current density than t-HTTH devices. These differences were due to the low-lying HOMO energy levels of 8821


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Chemistry of Materials the hosts (−5.9 ∼ −6.1 eV), which matched better with the tMTTH (−5.68 eV), thereby facilitating energy transfer from host to dopant. In contrast, the large HOMO energy gap between t-HTTH (−5.16 eV) and hosts blocked energy transfer and thus caused poor device performance. Devices with t-MTTH doped in different polarity hosts gave a maximum brightness (Lmax) of 9160−9310 cd m−2 at 13.6−14.8 V (1330−1340 mA cm−2) and ηext, ηc, and ηp were DPEPO (1.21%, 2.56 cd A−1, and 2.12 lm W1−), TCB (1.70%, 3.48 cd A−1, and 2.96 lm W1−), and mCP (1.30%, 3.0 cd A−1, and 2.38 lm W1−), respectively. Figure 6, panel d displays the EL emission spectra with two distinct emission bands: a blue band (normal) centered at 460 nm and a broad yellow/orange band (tautomer) with a peak at around 550−560 nm, which was unambiguously assigned to the proton-transfer tautomer emission resulting from ESIPT, such that the white spectra generated covered the whole visible spectrum. The emission peaks for both normal and tautomer luminescence bands of t-MTTH were affected by the host polarity with the CIE coordinates from (0.29, 0.31) to (0.29, 0.35), which resemble that of the PL spectra of t-MTTH in the host matrices (see Figure S9 in Supporting Information). In comparison to t-MTTH, devices using t-HTTH exhibited inferior performance with ηext of 0.29%−0.62% (Figure S12 in Supporting Information and Table 3). Furthermore, because of the weak tautomer emission, the EL spectra displayed normal emission with the CIE coordinates (0.24−0.26, 0.36−0.41). Color stability is an important parameter in WOLEDs and should be taken into serious consideration in evaluating the potential applications of new white lighting sources and display technologies. Unfortunately, many WOLEDs, except for a few with optimized device structure designs, show variation in color with the bias voltage or current, regardless of whether their structures are a single layer or a multilayer.50 In the current ESIPT system, both normal and proton-transfer tautomer emission in t-MTTH-based WOLEDs displayed highly stable chromaticity at various voltages (Figure 6e and Figure S13 in Supporting Information). This stable chromaticity may originate from the fast pre-equilibrium between the blue (normal excited state) and the yellow/orange (tautomer excited state) fluorophore, for which the small ΔE* is subject to negligible driving voltage dependence (vide supra).

together with basicity (proton acceptor)/acidity (proton donor) tuning of the system, makes feasible the harnessing of ESIPT dynamics/thermodynamics by the structural variation and medium polarity. Such versatilities are unprecedented among the ESIPT systems by facilitating the generation of white light. WOLEDs employing t-MTTH doped in TCB host featured unprecedented performance of ηext = 1.70%, 3.48 cd A−1, and 2.96 lm W−1 among all ESIPT systems. More importantly, the EL spectra showed superior color stability, with CIE coordinates of (0.29, 0.33) independent of luminance. The results demonstrate for the first time a measurable single molecule WOLED based on reversible ESIPT, which has farreaching implications for practical application.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04707. Materials and general methods, details synthetic procedures, and 1H and 13C NMR spectra; additional information for photophysical properties, computational approaches, and the OLED properties (PDF) X-ray crystallographic file for t-MTTM (CIF) X-ray crystallographic file for t-HTTH (CIF) X-ray crystallographic file for tMTTH (CIF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Pi-Tai Chou: 0000-0002-8925-7747 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS P.-T.C. and W.-Y.H. thank the Ministry of Science and Technology, Taiwan, for financial support.

3. CONCLUSION In summary, we have strategically designed and synthesized a new class of ESIPT molecules (t-HTTH and t-MTTH) bearing the TzTz core as the proton acceptor. As a result, normal (N* ≈ 440 nm) and tautomer (T* ≈ 560 nm) dual emission covering the entire visible region was observed for t-HTTH, tMTTH due to the existence of fast pre-equilibrium between the normal and tautomer excited states, which is firmly supported by femto-picosecond early relaxation dynamics. The dual emissions of these TzTz-based ESIPT fluorophores are attractive in this study. On the one hand, this ESIPT system displays charge-transfer behavior in the proton-transfer tautomer (T*, type II),22 which is much less common than the ordinary ESIPT systems. It turns out that the difference in dipole moments between the tautomer ground (T) and excited (T*) states is rather small. Therefore, the tautomer emission intensity can be fine-tuned by medium polarity in solution and polymer, free from the perturbation of the peak wavelength, manifesting the unique characteristic of this novel ESIPT system. On the other hand, the polarity sensitive ESIPT,

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Control of the Reversibility of Excited-State Intramolecular Proton

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